Power transmission cable

ABSTRACT

A power transmission cable comprising at least one power conductor, an insulating layer surrounding said conductor to form at least one insulated conductor, a flame-retardant halogen free protective sheath provided in a radially external position with respect to said insulated conductor, wherein said sheath has an inner and an outer layer in contact one another, said inner layer has a thickness at least equal to a thickness of said outer layer, the inner layer comprises a polymer material having a glass transition temperature equal to or lower than −30° C., and the outer layer comprises a mud resistant polymer material.

FIELD OF THE INVENTION

The present invention relates to a power transmission cable foroperating under chemically challenging conditions and at very lowtemperature.

Certain power cable applications, such as offshore, land rigs, marinevessels and oil and gas drilling rigs, require the cable to be protectedby an external sheath suitable to withstand mechanical stresses and/orharsh environmental conditions.

Such power transmission cable sheath should complies with variousrequirements.

In view of the environmental conditions where such cables have tooperate, a resistance to chemicals is required, such chemicals being,for example, sea water, hydrocarbons, oils, drilling fluids and mud.Power cable should be provided with a sheath chemically resistant to theattack of these substances, in accordance to national or internationalrecommendation such as NEK (Norsk Elektroteknisk Komite) 606 or IEC60092-359.

For health and safety reasons, such cables should qualify as low-smokezero-halogen, i.e. the covering layers thereof, such as insulating layerand sheath should emit limited smoke and no chlorine (the halogentypically present in covering compounds) when exposed to sources of heator fire.

Many applications find place in cold environment, as “cold” beingintended temperatures below −30° C. or more. Such cables should becapable to maintain the mechanical characteristics requested by the use,e.g. flexibility and impact resistance, even at such low temperature.

DESCRIPTION OF RELATED ART

U.S. Pat. No. 4,547,626 discloses a cable which is said to have improvedflame/fire and oil/abrasion resistant properties. The cable is halogenfree since the conductor insulation and all sheaths are of theself-extinguishing type. The outer protective shield include a polyestertape winding and a self-extinguishing sheath, as well as an optionalthin extruded sheath of nylon which effectively protects the cable coreagainst abrasion and damaging hydrocarbons like oil and drilling mud.Whereas the optional outer oil and abrasion resistant layer of nylon ishalogen free, the material in itself is combustible, but the layer is sothin (in order of 0.2-0.6 mm) that when placed on top of theself-extinguishing outer protective sheath it will not sustain a fire.

The Applicant observed that this outermost layer cannot effectivelyoperate at low temperatures because the glass transition temperature ofnylon is substantially higher than 0° C. So this layer is brittle andcracks at low temperatures, leaving the underlying layers withoutprotection against the cited chemicals.

U.S. Pat. No. 6,133,367 discloses a flame and oil resistant thermosetcomposition comprising a blend of

-   -   (a) 50-95 wt % relative to component (b) of an ethylene-vinyl        acetate copolymer having a vinyl acetate percentage of about        18-60 wt %; and    -   (b) 5-50 wt % of an ethylene-vinyl acetate-carbon monoxide        terpolymer having a vinyl acetate percentage of 18-35 wt %; a CO        percentage of 3-20 wt %; and    -   (c) wire and cable acceptable excipients, wherein at least one        cross-linking agent is included, and wherein a plasticizer is        not required as an acceptable excipient.

The Applicant faced the problem of providing a power transmission cablewith a sheath capable of withstanding chemical aggressions, especiallyfrom oil and drilling mud, and to preserve the mechanicalcharacteristics, such as flexibility and impact resistance, at very lowtemperatures (below −30° C.).

SUMMARY OF THE INVENTION

The Applicant found that a power transmission cable may be effectivelyprotected against aggressive chemicals and may be used even at very lowtemperatures by providing the cable with a flame-retardant halogen freesheath comprising an inner and an outer layer, the outer layer beingresistant to chemicals and the inner layer being endowed with physicalfeatures such to withstand very low temperatures, said inner layerhaving a thickness at least equal to the thickness of said outer layer.

As used herein the following expressions have the following meanings:

“Drilling mud” means a fluid complex mixture used in oil and natural gaswells and in exploration drilling rigs. Drilling mud may includebentonite clay (gel) barium sulfate (barite) and hematite, or can bebased on naphthenic compounds, esters, aromatic oils, olefins.

“Mud resistant” means the ability to withstand drilling mud as definedby proper recommendations such as NEK 606:2004.

“Glass transition temperature (Tg)” means the temperature below which apolymer changes from rubbery to glassy state. Such a temperature may bemeasured according to known techniques such as, for example, byDifferential Scanning Calorimetry (DSC).

“Flame retardant halogen-free” indicates a material capable to preventthe spread of combustion by a low rate of travel so the flame will notbe conveyed, said material having a halogen content lower than 5% byweight, as provided, for example, by IEC 60092-359 SHF2

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a power transmission cable comprising:

-   -   at least one power conductor;    -   an insulating layer surrounding said conductor to form at least        one insulated conductor;    -   a flame-retardant halogen free protective sheath provided in a        radially external position with respect to said insulated        conductor;        wherein:    -   said sheath has an inner and an outer layer in contact one        another,    -   said inner layer has a thickness at least equal to the thickness        of said outer layer,    -   the inner layer comprises a polymer material having a glass        transition temperature equal to or lower than −30° C.; and    -   the outer layer comprises a mud resistant polymer material.

For the purpose of the present description and of the claims whichfollow, except where otherwise indicated, all numbers expressingamounts, quantities, percentages, and so forth, are to be understood asbeing modified in all instances by the term “about”. Also, all rangesinclude any combination of the maximum and minimum points disclosed andinclude any intermediate ranges therein, which may or may not bespecifically enumerated herein.

Advantageously, said inner layer has a thickness of at least 1.5 timesthe thickness of the outer layer, more preferably 2 times the thicknessof the outer layer. The thickness of the inner layer can amount up to 20times the thickness of the outer layer.

Preferably, said inner layer has a thickness of from 1.0 mm to 10.0 mm.

Preferably, the polymer material of the inner layer is selected from:

-   -   a) an alkylene/vinyl acetate copolymer or a mixture of        alkylene/vinyl acetate copolymers having an average content of        vinyl acetate comonomer of from 20 to 50% by weight with respect        to the weight of the copolymer;    -   b) an alkylene/alkyl acrylate copolymer or a mixture of        alkylene/alkyl acrylate copolymers having an average content of        alkyl acrylate comonomer equal to or lower than 40% by weight        with respect to the weight of the copolymer.

Preferably the alkylene comonomer of copolymer a) or of copolymer b) isethylene comonomer.

More preferably, the average content of vinyl acetate comonomer in thecopolymer a) is of from 30% to 40% by weight with respect to the weightof the copolymer.

Advantageously, the alkyl acrylate of copolymer b) is selected frommethyl acrylate and butyl acrylate.

Preferably, the average content of alkyl acrylate comonomer in thecopolymer b) is equal to or higher than 20% by weight with respect tothe weight of the copolymer.

Preferably, the polymer material of the inner layer comprises from 40%to 80% by weight with respect to the weight of the polymer material of aflame-retardant filler.

Preferably the flame-retardant filler is selected from inorganic salts,oxides, hydroxides or mixture thereof. Magnesium hydroxide [Mg(OH)₂],aluminium hydroxide [Al(OH)₃], magnesium carbonate (MgCO₃) and themixtures thereof are preferred.

The magnesium hydroxide can be of natural origin, for example obtainedby grinding a mineral such as brucite, or of synthetic origin.

As used herein as “synthetic magnesium hydroxide” is intended amagnesium hydroxide in form of flattened hexagonal crystallitessubstantially uniform both in size and morphology. Such a product may beobtained by various synthetic routes involving the addition of alkalisto an aqueous solution of a magnesium salt and subsequent precipitationof the hydroxide by heating at high pressure (see for example U.S. Pat.No. 4,098,762 or EP-780,425 or U.S. Pat. No. 4,145,404).

The polymer material of the inner layer can comprise additives such asthermal and oxidative stabilizing agents, peroxides, antioxidants, resinmodifiers and the like.

Preferably said outer layer has a thickness of from 0.5 mm to 5.0 mm.

Preferably the polymer material of the outer layer is an alkylene/alkylacrylate copolymer or a mixture of alkylene/alkyl acrylate copolymershaving an average content of alkyl acrylate comonomer equal to or higherthan 40% by weight with respect to the weight of the copolymer/s.

More preferably, the average content of alkyl acrylate comonomer isequal to or higher than 50% by weight with respect to the weight of thecopolymer/s. The average content alkyl acrylate comonomer can amount to80% by weight with respect to the weight of the copolymer/s.

Preferably the alkylene comonomer of copolymer is an ethylene comonomer.

Advantageously the alkyl acrylate comonomer is selected from methylacrylate and butyl acrylate.

Advantageously, the polymer material of the outer layer has a Tg equalto or lower than −20° C.

In a preferred embodiment the outer layer comprises a flame retardantfiller. The kind and amount of said filler can be similar to those ofthe flame retardant filler of the inner layer.

In a preferred embodiment, the cable of the present invention comprisesa tape provided in a radially internal position with respect to thesheath. Advantageously said tape is helically wound around the insulatedconductor so as to have overlapping coils. In other words, nointerstices are provided such to put the inner layer and the underlyinglayers into contact.

Advantageously, said tape is made of a material selected from polyamideand polyester.

Advantageously, said tape is in form of textile material, preferablyembedded in a polymeric matrix.

Preferably, the polymeric matrix where the textile tape is embedded inis based on an elastomeric polymer, for example selected from naturalrubber (NR), styrene-butadiene rubber (SBR), butyl rubber (BR), ethylenepropylene diene monomer rubber (EPDM), ethyl vinyl acetate rubber (EVA).

These and further features of the invention will become apparent fromFIG. 1 shown herein below and from the subsequent examples.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a cross-section of a power transmission cable according toa first embodiment of the invention;

FIG. 2 shows a cross-section of a power transmission cable according toa second embodiment of the invention.

Cable 100 of FIG. 1 is a medium-voltage and comprises three conductors1, each surrounded by an insulating layer 2 to provide three insulatedconductors 1,2.

The term “medium voltage” indicates a voltage of from 1 kV to 35 kV.

The insulated conductors 1,2 stranded together and, optionally wrappedby a tape, e.g. in paper or textile material (not shown).

The twisting of the insulated conductors 1,2 gives rise to a pluralityof voids, i.e. interstitial zones, which, in a transverse cross sectionalong the longitudinal length of the strand, define an externalperimeter profile of the latter of non-circular type.

Therefore, in order to allow the correct application of the radiallyexternal layers in a position radially external to said stranding, abedding 3 a polymeric material (for example, an elastomeric mixture), isapplied by extrusion to fill said interstitial zones so as to confer tothe stranding a substantially even transverse cross section, preferablyof the circular type.

In the presently depicted cable 100, the bedding 3 is surrounded by anarmour 4, for example in form of copper braids, or in polymeric textilematerial.

The armour 4 of FIG. 1 is in turn surrounded by a sheath comprising aninner layer 5 and an outer layer 6.

The cable 200 of FIG. 2 is similar to that of FIG. 1, thus the samereference number are used for the shared components thereof. Cable 200lacks an armour.

The sheath of cable 200 comprises an inner layer 5, an outer layer 6 anda tape 7 provided in a radially internal position with respect to theinner layer 5. In the present case, the tape 7 is provided to surroundthe bedding 3.

The inner layer 5 and the outer layer 6 are in close contact oneanother. This close contact is preferably obtained by extrusion of theouter layer 6 on the inner layer 5 or by co-extrusion of a sheath formedby an inner layer 5 and an outer layer 6.

Example 1 and Comparative Example 2

The inner layer of a power transmission cable according to the inventionwas obtained by extrusion of a polymer composition according to Table 1.

TABLE 1 Ingredients phr Percent by weight ELVAX 40 ® L-03 50.0 24.2ELVAX ® 265 47.0 22.8 HYDROFY ® GS 1.5 34.0 16.5 MARTINAL ® OL 107 LE67.0 32.5 Antioxidant agent 1.5 0.7 Peroxide 2.2 1.1 Other additives 4.52.2 Total 206.2 100.0 Elvax ® 40L-03 = ethylene/vinyl acetate copolymerwith a vinyl acetate comonomer content of 40% by weight; glasstransition temperature of −32° C. (marketed by DuPont); Elvax ® 265 =ethylene/vinyl acetate copolymer with a vinyl acetate comonomer contentof 28% by weight; glass transition temperature of −5° C. (marketed byDuPont); Hydrofy ® G-1.5 = natural magnesium hydroxide powders obtainedby grinding brucite, marketed by Nuova Sima Srl; Martinal ® OL-107 LE =aluminium hydroxide, marketed by Albemarle;

The admixture of the two ethylene/vinyl acetate copolymers provided amixture having an amount of vinyl acetate comonomer of 35% by weight anda glass transition temperature of −34° C.

The inner layer of a power transmission cable provided as comparison wasobtained by extrusion of a polymer composition according to Table 2.

TABLE 2 Ingredients Phr Percent by weight LEVAPREN ®600 HV 100.0 47.6HYDROFY ® GS 1.0 32.9 15.6 MARTINAL ® OL 107 LE 67.1 31.9 Antioxidantagent 1.4 0.70 Peroxide 5.5 2.6 Other additives 3.4 1.6 Total 210.3100.0 Levapren ®600 HV = ethylene/vinyl acetate copolymer with a vinylacetate comonomer content of 60% by weight; glass transition temperatureof −26° C. (marketed by Lanxess); Hydrofy ® G-1.0 = natural magnesiumhydroxide powders obtained by grinding brucite, marketed by Nuova SimaSrl; Martinal ® OL-107 LE = aluminium hydroxide, marketed by Albemarle.

Example 3 and Comparative Example 4

The outer layer of a power transmission cable according to the inventionwas obtained by extrusion of a polymer composition according to Table 3.

TABLE 3 Ingredients Parts by weight Percent by weight VAMAC ® DP 95.041.1 KISUMA 5-A 60.0 26.0 MARTINAL ® OL 107 LE 60.0 26.0 Antioxidant 1.40.60 Peroxide 2.4 1.0 Other additives 8.4 5.3 Total 230.9 100.0 Vamac ®DP = ethylene/methyl acrylate copolymer with a content of methylacrylate comonomer of 58% by weight; glass transition temperature of−29° C. (marketed by DuPont); Kisuma ® 5-A = precipitated magnesiumhydroxide (marketed by Kyowa Chemical Industry); Martinal ® OL-107 LE =aluminium hydroxide, marketed by Albemarle.

The outer layer of a power transmission cable provided as comparison wasobtained by extrusion of a polymer composition according to Table 4.

TABLE 4 Ingredients Parts by weight Percent by weight Levapren ® 800 HV100.0 32.5 Brucite SFP + MARTINAL ® 130.0 42.2 OL104 LE Frimiz MZ-1 69.822.6 antioxidant 1.0 0.4 peroxide 2.5 0.8 Other additives 4.7 1.5 Total308.0 100.0 Levapren ® 800 HV: ethylene/vinyl acetate copolymer with avinyl acetate comonomer content of 80% by weight; glass transitiontemperature of −3° C. (marketed by Lanxess); Brucite SFP: naturalmagnesium hydroxide obtained by grinding brucite Martinal ® OL-104 LE =aluminium hydroxide (marketed by Albemarle) Frimiz MZ-1 = magnesiumcarbonate (marketed by Alpha Calcit Fullstoff GmbH & CO).

Example 5

Three cables were manufactured with a sheath composed by an inner layer3.0 mm-thick and an outer layer 1.5 mm-thick, said inner and outer layerbeing as follows:

Cable 1: inner layer of Example 1 and outer layer of Example 3;

Cable 2: inner layer of Example 1 and outer layer of Example 4;

Cable 3: inner layer of Example 2 and outer layer of Example 3.

Cables 1 is according to the invention, while Cables 2 and 3 areprovided as comparison.

Each cable was tested according to CSA (Canadian Standards Association)C22.2 No. 0.3-01 (2001) to check the cable response at an impact of ahammer head (weight=1.36 kg) after cooling to −40° C. for 4 hours.

After the test, Cable 1 according to the invention showed no cracks orruptures. The polymeric material of the inner layer has a glasstransition temperature such to confer the layer the capability to absorbthe impact exerted on the sheath without damages to the outer layer madeof a polymeric material with a higher glass transition temperature.

Cable 2, wherein the inner layer of the sheath is made of a polymermaterial having a glass transition temperature lower than −30° C.(Example 1), but the outer layer has a glass transition temperaturehigher than −20° C. (Example 4), showed cracks in the outer layer afterthe impact test. This result indicates that in spite of the presence ofan inner layer with a very low glass transition temperature, the outerlayer of the sheath cannot stand the impact when said outer layer ismade of a material with a glass transition temperature just below 0° C.as a consequence, a cable like Cable 2 cannot be used, for example, indrilling activities located in very cold environment, because the cracksof the mud-resistant outer layer let the inner layer (not mud-resistant)prone to the chemical attack of the mud.

Cable 3, wherein the outer layer of the sheath is made of a polymericmaterial having has a glass transition temperature lower than −20° C.(Example 3), but the inner layer is made of a polymeric material havinga glass transition temperature higher than −30° C. (Example 4), showedcracks and ruptures in both the layers. This result indicates that whenan outer layer with a low glass transition temperature is not supportedby an inner layer suitable for retaining the mechanical characteristicthereof at very low temperatures, said outer layer cannot withstandimpact at such temperatures, thus depriving the inner layer (and otherlayers provided in a radially internal position) of the protectionagainst the chemical attack of the mud. Again, a cable as Cable 3 cannotbe used, for example, in drilling activities located in very coldenvironment, because the cracks of the mud-resistant outer layer let theinner layer (not mud-resistant) prone to the chemical attack of the mud.

1. Power transmission cable comprising: at least one power conductor; aninsulating layer surrounding said conductor to form at least oneinsulated conductor; a flame-retardant halogen free protective sheathprovided in a radially external position with respect to said insulatedconductor; wherein: said sheath has an inner and an outer layer incontact one another, said inner layer has a thickness at least equal toa thickness of said outer layer, the inner layer comprises a polymermaterial having a glass transition temperature equal to or lower than−30° C.; and the outer layer comprises a mud resistant polymer material.2. Power transmission cable according to claim 1 wherein said inner hasa thickness of at least 1.5 times the thickness of the outer layer. 3.Power transmission cable according to claim 2 wherein, said inner has athickness 2 times the thickness of the outer layer.
 4. Powertransmission cable according to claim 1 wherein said inner layer has athickness of from of from 1.0 mm to 10.0 mm.
 5. Power transmission cableaccording to claim 1 wherein the polymer material of the inner layer isselected from: a) an alkylene/vinyl acetate copolymer or a mixture ofalkylene/vinyl acetate copolymers having an average content of vinylacetate comonomer of from 20 to 50% by weight with respect to the weightof the copolymer; b) an alkylene/alkyl acrylate copolymer or a mixtureof alkylene/alkyl acrylate copolymers having an average content of alkylacrylate comonomer equal to or lower than 40% by weight with respect tothe weight of the copolymer.
 6. Power transmission cable according toclaim 5 wherein the alkylene of copolymer a) or of copolymer b) is anethylene comonomer.
 7. Power transmission cable according to claim 5wherein the content of vinyl acetate comonomer in the copolymer a) is offrom 25% to 45% by weight with respect to the weight of the copolymer.8. Power transmission cable according to claim 5 wherein the alkylacrylate of copolymer b) is selected from methyl acrylate and butylacrylate.
 9. Power transmission cable according to claim 5 wherein thecontent of alkyl acrylate comonomer in the copolymer b) is equal to orhigher than 20% by weight with respect to the weight of the copolymer.10. Power transmission cable according to claim 1 wherein the polymermaterial of the inner layer comprises from 40% to 70% by weight withrespect to the weight of the polymer material of a flame-retardantfiller.
 11. Power transmission cable according to claim 10 wherein theflame-retardant filler is selected from inorganic oxides and hydroxidesor mixture thereof.
 12. Power transmission cable according to claim 11wherein the flame-retardant filler is selected from magnesium hydroxide,aluminium hydroxide and the mixtures thereof.
 13. Power transmissioncable according to claim 1 wherein said outer layer has a thickness offrom 0.5 mm. to 5.0 mm.
 14. Power transmission cable according to claim1 wherein the polymer material of the outer layer is an alkylene/alkylacrylate copolymer or a mixture of alkylene/alkyl acrylate copolymershaving an average content of alkyl acrylate comonomer equal to or higherthan 40% by weight with respect to the weight of the copolymer/s. 15.Power transmission cable according to claim 14 wherein the averagecontent of alkyl acrylate comonomer is equal to or higher than 50% byweight with respect to the weight of the copolymer/s.
 16. Powertransmission cable according to claim 14 wherein the alkylene is anethylene comonomer.
 17. Power transmission cable according to claim 14wherein the alkyl acrylate comonomer is selected from methyl acrylateand butyl acrylate.
 18. Power transmission cable according to claim 1wherein the polymer material of the outer layer has a Tg equal to orlower than −20° C.
 19. Power transmission cable according to claim 1having a tape provided in a radially internal position with respect tothe sheath.
 20. Power transmission cable according to claim 19 whereinsaid tape is made of a material selected from polyamide and polyester.21. Power transmission cable according to claim 19 wherein said tape isin form of textile material.
 22. Power transmission cable according toclaim 21 wherein, said textile material is embedded in a polymericmatrix.
 23. Power transmission cable according to claim 22 wherein thepolymeric matrix is based on a polymer.